1. Field of the Invention
The invention generally relates to communications systems. In particular, the invention relates to linearizing radio frequency (RF) power amplifiers.
2. Description of the Related Art
Radio frequency (RF) power amplifiers are widely used to transmit signals in communications systems. Typically, a signal to be transmitted is concentrated around a particular carrier frequency that occupies a defined channel. Information is provided in the form of a modulation of amplitude, phase, frequency, or some combination of these, which causes the information to be represented by energy spread over a band of frequencies around the carrier frequency. In many schemes, the carrier itself is not sent since it is not essential to the communication of the information.
When an amplifier amplifies a signal that contains amplitude variations, the signal will become distorted if the amplifier does not exhibit a linear amplitude and phase transfer characteristic. This means that the output of the amplifier is not linearly proportional to the input of the amplifier. It will also suffer distortion if the phase shift introduced by the amplifier is not linear over the range of frequencies present in the signal or if the phase shift caused by the amplifier varies with the amplitude of the input signal. The distortion introduced includes inter-modulation of the components of the input signal. The products of the inter-modulation can appear within the bandwidth of the signal causing undesirable interference. They can also extend outside the bandwidth originally occupied by the signal. This can cause interference in adjacent channels and violate transmitter licensing and regulatory spectral emission requirements. Although filtering can be used to remove the unwanted out-of-band distortion, filtering is not always practical, especially when the amplifier is operates on several different frequencies.
Distortion products that are at multiples of the carrier frequency can also be produced in a non-linear amplifier, but these are relatively easy to remove by filtering. Inter-modulation is also a problem when multiple signals are amplified in the same amplifier even if individually, they do not have amplitude variations. This is because the combination of the multiple signals produces amplitude variations as the various components beat with each other by adding and subtracting as their phase relationships change.
Even well-designed amplifiers can introduce some distortion. In practice, perfect linearity over a wide range of amplitude is difficult to realize. In addition, as any amplifier nears its maximum output power capacity, the output no longer increases as the input increases. At this point, the amplifier is not regarded as linear. A typical amplifier becomes significantly non-linear at a relatively small fraction of its maximum output capacity. In order to maintain linearity, an amplifier is often operated at an input and output amplitude that is low enough such that the signals to be amplified are within a part of the amplifier's transfer characteristic that is substantially linear. This is a method of operation, described as “backed off,” in which the amplifier has a relatively low supplied-power-to-transmitted-power conversion efficiency. A “Class A” amplifier operating in this mode may be linear enough to transmit a signal relatively cleanly, but might typically be only about 1% efficient. This wastes power and means that the amplifier has to be large and relatively expensive. It also means that the waste power is dissipated as heat, which has to be removed by relatively bulky and expensive cooling systems.
Communication schemes using signals that have constant amplitude with frequency and phase modulation can use relatively non-linear amplifiers. These types of signals are relatively immune to the effects of distortion, and the corresponding amplifiers can be smaller, cooler running, more power efficient, and less expensive than “Class A” amplifiers. For example, modulation of this type is used in conventional radio paging systems, which use continuous phase frequency shift keying (CPFSK) modulation.
Many of the newer, bandwidth-efficient modulation schemes have both amplitude and phase variations. There is also frequently a desire to be able to transmit multiple signals on different channels through a single amplifier. This reduces the number of separate amplifiers required and avoids the need for relatively large and costly high level output signal combining filters, which can incur undesirable power losses.
Digital Predistortion
Conventional digital cellular telephony services employ linear modulation schemes to encode baseband information in both the amplitude and phase of the RF carrier. This is undertaken to achieve an increase in spectral efficiency. In a conventional RF amplifier, if significant inter-modulation and distortion products are to be avoided, Class A linear amplifiers should be employed. However, relatively high-power linear amplifiers are generally inefficient and undesirable in a system where cost and heat dissipation are prohibitive factors, e.g., a cellular telephone basestation. To avoid the compromise of constraints between the regulatory spectral emission mask and amplifier efficiency, attempts have been made to harness the efficiency of non-linear Class AB amplifiers by various linearization techniques.
Experimental analog feedback techniques are disadvantageously limited to relatively narrow operating bandwidths, are extremely sensitive to amplifier variations, and are prone to instability. See A. Bateman, D. M. Haines and R. J. Wilkinson, Linear Transceiver Architectures, IEEE Proc. Veh. Technology. Conf., Philadelphia, Pa. 1988, IEEE Catalog 2622-9/88/0000-0478, pp. 478-484. Also see R. D. Stewart and F. F. Tusubira, Feedforward Linearization of 950 MHz Amplifiers, Inst. Elec. Eng. Proc., Vol. 1, pt H, No. 5, pp. 347-350, October 1988.
Consequently, these amplifier configurations are not appropriate for mass production. Simulation work has been presented that postulates the advantage of employing adaptive digital feedback for the predistortion at baseband. See J. K. Cavers, Amplifier Linearization Using A Digital Predistorter With Fast Adaptation And Low Memory Requirements, IEEE Trans Veh. Technol., Vol. 39, pp. 374-383, November 1990. Also see Y. Nagata, Linear Amplification Technique For Digital Mobile Communications, in proc. IEEE Veh. Technology. Conf., San Francisco, Calif., 1989, pp. 159-164. Such simulation work has promised excellent reductions in out-of-band spectral emissions, typically in excess of 25 dB. These techniques are relatively insensitive to amplifier variations and provide an attractive design that is suitable for mass production.